It has long been known in the art to form single ion conducting polymer electrolyte membranes and gels from organic polymers known as ionomers which contain ionic pendant groups. Such membranes and gels are useful in electro-chemical applications such as batteries, electrolyzers, and fuel cells. In widespread commercial use are Nafion® membranes available from E. I. du Pont de Nemours and Company. Nafion® is formed by copolymerizing tetra-fluoro ethylene (TFE) with perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride), as disclosed in U.S. Pat. No. 3,282,875. Also known are copolymers of TFE with perfluoro (3-oxa-4-pentene sulfonyl fluoride), as disclosed in U.S. Pat. No. 4,358,545. The copolymers so formed are converted to the ionomeric form by hydrolysis, typically by exposure to an appropriate aqueous base, as disclosed in U.S. Pat. No. 3,282,875. Hydrogen, lithium, sodium and potassium are all well known in the art as suitable cations for the above cited ionomers.
It will be appreciated by one of skill in the art that the higher the concentration of ionically conductive moieties in said membranes or gels (that is, the lower the equivalent weight, EW, of the associated ionomer) the higher the conductivity of the membrane, and the better the electrochemical performance of the cell in which it is incorporated.
In the art of direct methanol fuel cells (DMFCs) it is found that the membranes are highly susceptible to “methanol crossover”—diffusion of methanol across the membrane into the oxygen-rich section—which degrades cell performance. It is further recognized in the art that within a given family of ionomers, methanol crossover is worst for low EW membranes—the very membranes which are desired for highest conductivity.
One approach to the problem has been to fabricate layered membrane structures, alternating between high and low EW membranes in an attempt to retard methanol crossover without sacrificing too much in the way of conductivity.
Mauritz et al, Multiphase Polymer Materials: Ionomers and Blends, ACS Symposium Series #395, Utracki and Weiss (ed.), Chapter 16, American Chemical Society, Washington D.C., 1989, and Deng et al., J. Appl. Polym. Sci, 68, 747-763 (1998) disclose a method for forming perfluoroionomer membrane-based nanocomposites by dissolving solutions of tetra-alkoxysilanes, alkylalkoxysilanes, and mixtures thereof in solvent swollen perfluorosulfonic acid membranes, followed by in situ hydrolysis, followed in turn by condensation and drying to form domains of approximately 5 nm or less in scale of organically modified silica homogeneously dispersed in the perfluorosulfonic acid membrane at a concentration of approximately 10% by weight. The resultant membranes are said to have potential utility in fuel cells. The membranes exhibit reduced methanol uptake compared to a control.
Beckerbauer et al., U.S. Pat. No. 5,958,822, discloses a method for synthesizing modified silanes having the formula XnR13-nSiR2RfSO2F wherein R1 is a non-hydrolyzable group such as alkyl or acyl, X is a hydrolyzable group including halogen, alkoxy, or acyloxy, R2 is an alkylene radical having at least two carbons linearly between Si and Rf, Rf is a perfluoroalkylene ether radical, and n=1 to 3. The sulfonyl fluoride functionalized silane compositions thus produced may then be hydrolyzed by methods therein disclosed to a composition having the formula (HO)3SiR2RfSO3K, which is readily converted to the sulfonic acid by acid exchange. The acid form is then applied to a surface to serve as a supported acid catalyst.
In Harmer et al, Chem. Commun., 1997, pp. 1803f, the hydrolyzed silane sulfonate of Beckerbauer, op. cit., is co-condensed with a tetralkoxysilane in a sol-gel process to form a functionalized silica network. One, two and three-dimensional networks formed by sol-gel processes from alkoxysilanes co-condensed with organically modifed alkoxy silanes are known in the art. See, for example, Li et al., Adv.Mater. 11, 730-734, (1999), a review paper by Clark and Macquarrie, Chem. Commun. 1998, 853-860, and Harmer et al., Chem. Comm. 1997, 1803-1804. Harmer, in particular, provides a method for synthesizing (OC2H4)3Si(CH2)3(CF2)2O(CF2)2SO2F, hydrolyzing and co-condensing with a solution of hydrolyzed tetramethylsiloxane to form a form a fluorosulfonic acid functionalized silica network useful in catalysis.
Watanabe et al., U.S. Pat. No. 5,766,787, discloses a Nafion® perfluorosulfonic acid membrane formed as a composite containing both a catalyst such as platinum, and up to 50%, but in practice, only 5%, by weight of an oxide such as silica. The silica is in the form of a dense particulate of approximately 7 nm particle size. The composite membrane is said to be useful in fuel cells by reducing the requirements for humidifying the gaseous reactants, with the oxide specifically providing an improvement over the metal oxide-free catalyst/Nafion® composite by improving water retention. Fuel cell performance of a catalyst-free, metal oxide free Nafion® membrane was superior to the claimed membrane when humidified feed was employed. Further, the elimination of humidification of the feed resulted in >50% degradation of performance for both claimed and prior art membranes.